In many applications it is desirable to convert continuous, i.e., analog, information, such as the angle of a shaft, into a digital form to allow processing by a digital computer. For example, a typical device, such as a resolver, provides signal outputs which are representative of a shaft angle and it is desirable to convert such information to digital form, the digital information thereby accurately tracking the analog information from the resolver.
One approach to such process is described in U.S. Pat. No. 3,667,031, issued on May 30, 1972 to Cox et al. In accordance therewith, the output of a resolver is continuously tracked by utilizing an analog, phase-locked feedback loop which tracks the modulated resolver output wave forms in a highly accurate manner. In a dual-input form, the analog, phase-locked loop incorporates two phase-sensitive detectors, each detector operating on one of the output signals from the windings of the resolver, and the circuit is configured so as to provide resolver-angle to phase-angle conversion is an improved manner. Such a system reduces the conversion errors which arise due to resolver excitation amplitude mismatches and to phase-detector gain mismatches. Further, such a system reduces errors which are due to resolver input signals being out of exact quadrature.
A disadvantage of such a system, however, lies in the high costs of the components thereof which costs make the use of such a dual-input analog, phase-locked loop system too expensive for many applications. It is desirable, therefore, to design a system which will provide accurate resolver output tracking at much less cost than that of the system disclosed in the aforementioned Cox et al. patent.